Episcopic image projection systems are distinguished from diascopic image projection systems in which light passes through a graphic sheet for projection onto a screen. Because light in the episcopic system does not have to pass through the graphic sheet, opaque materials rather than transparent graphics can be used in projecting an image. For this reason, episcopic image projection systems are commonly referred to as opaque projectors.
Opaque projectors have the inherent advantage that plain paper graphics, and even three-dimensional objects, can be projected without any special preparation or special materials. This, of course, eliminates the time and expense needed to prepare transparencies which are required in diascopic image systems.
Attempts have been made to produce a compact, lightweight opaque projector which is capable of achieving excellent optical results. One such successful attempt is disclosed in the patent to Montgomery, U.S. Pat. No. 4,468,105, which is assigned to the assignee herein. The '105 projector is compact and portable, enabling it to be readily stored and transported. It is relatively uncomplicated to assemble and use.
An improvement on this opaque projector is disclosed in pending U.S. pat. app. Ser. No. 704,398, filed Feb. 22, 1985, also assigned to the assignee herein. That application is particularly directed to an improved assembly for more stable mounting of the reflecting mirror used in the projector, which further serves to minimize the size of the mirror used as well as to improve the quality of the projected image. The disclosure of pending U.S. pat. app. Ser. No. 704,398 is hereby incorporated in this application by reference, as if specifically set forth herein.
The present invention reflects yet a further improvement upon the type of lightweight, portable and compact opaque projector shown in the foregoing patent disclosures. The particular problem addressed by the present invention is an improvement in the manner of dissipating, in a controlled fashion, the intense heat generated within the illumination chamber in the interior of the opaque projector, while maintaining the exterior of the projector at moderate temperatures, i.e., not uncomfortable to the touch.
In order to achieve a bright projected image in an opaque projector, which relies upon reflected rather than transmitted light, it is typically necessary to use an extremely bright light source to illuminate the opaque graphic, such as a plurality of high intensity lamps. Such lamps ordinarily provide considerable infrared as well as visible light radiation, and a significant portion of this radiation is absorbed by the interior housing of the projector which surrounds the illumination chamber. Heat is also generated by resistance heating of the lamps themselves. Heat absorption by the interior of the projector is aggravated by the fact that light and heat-absorptive coatings, e.g. black surfaces, are typically used on the interior of the projector to minimize stray light reflections that could distort the projected image, or spill into the room in which the projector is being used thereby creating undesirable stray illumination.
Adequate ventilation of the interior of the projector must therefore be provided to prevent undesirable heating of the projector, which could render it uncomfortable to the touch, as well as shorten the useful life of its components. A large movement of air through the illumination chamber can be provided to carry away the excess heat, but there are problems with such a solution, particularly where a compact opaque projector is involved. For example, vigorous air currents can cause the target graphic to be blown about the interior of the projector, or otherwise disturbed during display, requiring some means to anchor the graphic, such as placing it under a sheet of glass. This of course adds complexity to the use of the projector, restricts use of three-dimensional objects and further detracts from portability by adding weight.
It is also known to utilize a cooling air plenum adjacent the exterior surface of an optical projector for conducting a flow of cooling air around the projector. Such prior art air plenum systems are not readily adaptable to a lightweight portable opaque projector of the type under consideration herein, however.